Light emitting device with microlens array

ABSTRACT

A method of manufacturing a micro-lens array and light-emitting device, including forming a first structured polymer film with close packed surface cavities having a mean diameter of less than 20 micrometers and a relatively lower surface energy surface, forming a transparent second structured film with an array of microlenses formed thereon corresponding to the cavities of the first structured film, wherein the second structured film includes a relatively high surface energy material and has a refractive index greater than 1.45, and wherein the microlenses are randomly distributed, separating the second structured film with the micro-lens array from the first structured polymer film, and attaching the second structured film to a transparent substrate or cover of a light-emitting device through which light is emitted. Use of microlens arrays formed from relatively high surface energy materials enables matching refractive index of microlens array to that of light-emitting devices substrate or cover through which light is emitted and relatively high elastic modulus providing good scratch resistance.

FIELD OF THE INVENTION

This invention relates to a light-emitting device having a micro-lensarray, and more particularly to self-emissive light-emitting deviceshaving a high fill-factor micro-lens array in optical contact with atransparent substrate or cover of the light-emitting device, and amethod of fabricating the high fill-factor micro-lens array.

BACKGROUND OF THE INVENTION

Light-emitting devices comprising self-emissive thin film light emittingelements such as organic light emitting diodes (OLEDs) represent anattractive technology for flat panel display and solid-state lighting.OLED devices generally can have two formats known as small moleculedevices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLEDdevices such as disclosed in U.S. Pat. No. 5,247,190. Either type ofOLED device may include, in sequence, an anode, an organic EL element,and a cathode. The organic EL element disposed between the anode and thecathode commonly includes an organic hole-transporting layer (HTL), anemissive layer (EL) and an organic electron-transporting layer (ETL).Holes and electrons recombine and emit light in the EL layer. Tang etal. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65,3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficientOLEDs using such a layer structure. Since then, numerous OLEDs withalternative layer structures, including polymeric materials, have beendisclosed and device performance has been improved.

Light is generated in an OLED device when electrons and holes that areinjected from the cathode and anode, respectively, flow through theelectron transport layer and the hole transport layer and recombine inthe emissive layer. Many factors determine the efficiency of this lightgenerating process. For example, the selection of anode and cathodematerials can determine how efficiently the electrons and holes areinjected into the device; the selection of ETL and HTL can determine howefficiently the electrons and holes are transported in the device, andthe selection of EL can determine how efficiently the electrons andholes be recombined and result in the emission of light, etc. It hasbeen found, however, that one of the key factors that limits theefficiency of OLED devices is the inefficiency in extracting the photonsgenerated by the electron-hole recombination out of the OLED devices.Due to the high optical indices of the organic materials used, most ofthe photons generated by the recombination process are actually trappedin the devices due to total internal reflection. These trapped photonsnever leave the OLED devices and make no contribution to the lightoutput from these devices.

A typical OLED device uses a glass substrate, a transparent conductinganode such as indium-tin-oxide (ITO), a stack of organic layers, and areflective cathode layer. Light generated from the device is emittedthrough the glass substrate. This is commonly referred to as abottom-emitting device. Alternatively, a device can include a substrate,a reflective anode, a stack of organic layers, a top transparent cathodelayer, and a transparent encapsulating cover. Light generated from thedevice is emitted through the top transparent electrode andencapsulating cover. This is commonly referred to as a top-emittingdevice. In these typical devices, the index of the ITO layer, theorganic layers, and the glass is about 2.0, 1.7, and 1.5 respectively.It has been estimated that nearly 60% of the generated light is trappedby internal reflection in the ITO/organic EL element, 20% is trapped inthe glass substrate, and only about 20% of the generated light isactually emitted from the device and performs useful functions. Thus, itcan be seen that a major shortcoming of OLEDs is that only a smallfraction of light generated in the organic layers is emitted from thedevice. A significant amount of light is trapped by total internalreflection (TIR) because of the relatively large differences inrefractive index at the anode-substrate and substrate-air interfaces.

Methods for improving the extraction or out-coupling of light from OLEDsare known in the art. A number of approaches have focused on thesubstrate-air interface. For example, Moller and Forrest (Journal ofApplied Physics, volume 91, page 3324, March 2002 incorporated here asreference) have demonstrated an increase in light output by a factor or1.5 by attaching a micro-lens array to the glass substrate of an OLED.But there are significant problems with this approach. Preparation ofthe micro-lens array is by a complex multi-step process involvingchemical vapor deposition, photolithography and chemical etching. Also,the micro-lenses do not have the optimum shape. Calculations by Peng etal. (Journal of Display Technology, volume 1, page 278, December 2005incorporated here as reference) indicate that micro-lenses should haveperfectly hemispherical shape for maximum light extraction. Peng et al.describe a process for fabrication of a micro-lens array based oncoating a thin layer of photo-resist material on a glass substratefollowed by patterning the photo-resist by conventional lithography andthen modifying the shape of the photo-resist disks by melting andre-flow. Although the process is an improvement over the method ofMoller and Forrest that is reflected in higher light extraction, afactor of 1.85 versus 1.5; there are still problems to be overcome.Melting and re-flow of the photo-resist is difficult to controlresulting in significant deviations from hemispherical shape.Furthermore, the distance between lenses is fixed at 1 μm because asmaller spacing or higher resolution is difficult to obtain using thisprocess that is characteristic of “top-down” or conventionalmicro-fabrication technology. The minimum spacing of 1 μm limits thearea fill factor to less than 0.80 for a hexagonally close packed arrayof hemispherical micro-lenses having diameters of less than 20micrometers, where the fill factor is defined as the ratio of the areaoccupied by the micro-lenses to the total area of the surface. Clearly,it is desirable to have a micro-lens array with fill factor close tounity to achieve maximum light extraction.

US2004/0189185 also teaches an OLED device with a micro-lens array.However, once again the micro-lens array is fabricated by conventionalmicro-fabrication methods such as wet etching and photo-resist re-flowthat have the same disadvantages as noted above.

Sun and Forrest (Journal of Applied Physics, volume 91, pages 073106-1to 073106-6, published online Oct. 11, 2006) describes an OLED devicewith microlenses fabricated by imprint lithography, wherein a negativemicrolens array pattern is etched into a glass mold, which is used toimprint a microlens array in a polymethylmethacrylate layer spun-coatedon an OLED glass substrate. The described process enables a microlensarray to be formed with PMMA, which has a refractive index closelymatched to the glass substrate, and a desirably high elastic modulusproviding improved scratch resistance compared to previously describedmicrolens arrays formed from PDMS. Formation of a close packed hexagonalarray consisting of 6.6 micrometer diameter by 2.2 micrometer highmicrolenses is reported.

Yabu and Shimomura (Langmuir, volume 21, page 1709, 2005) describe analternative approach for preparing micro-lens arrays. In this process, asolution of polymer in a volatile organic solvent is cast under humidconditions. Evaporation of the organic solvent under the same humidconditions followed by subsequent evaporation of condensed waterdroplets from the cast composition results in a polymer film containinga uniform closely packed three dimensional network of spherical pores. Aclose-packed array of pillar structures is then generated by peeling offthe top layer of the film with spherical pores. A polymeric material issubsequently coated over the pillar structure, cured and then releasedto form a micro-lens array. Yabu and Shimomura do not quantify the fillfactor or the shapes of the individual micro-lenses in the array.Furthermore, they do not discuss the effectiveness of the micro-arrayfor light extraction from OLEDs. Also, fabrication of the array involvesa large number of steps that may not be suitable for low-cost highvolume manufacturing. A simple process requiring less number of stepsleading to micro-lens arrays having a high fill factor and hemisphericalshaped micro-lenses and the integration of such arrays with OLEDs isstill needed.

An additional problem with the micro-lens array of Yabu and Shimomuraand other micro-lens arrays in the prior art is that the micro-lensarray comprises a precisely ordered array of lenses. An ordered array oflenses in an OLED display can cause significant diffractive artifactsfrom intense ambient point sources, such as sunlight, or incandescentlamps. It would be desirable to have OLED devices with integratedmicro-lens arrays wherein the lenses in the micro-lens arrays areclose-packed but randomly distributed.

Srinivasarao et al. (Science, volume 292, page 79, 2001) also describe aprocess for creating a micro-voided polymer film that involves casting asolution of polymer in a volatile organic solvent in the presence ofmoist air. Srinivasarao et al. indicate that the shape of themicro-voids in the polymer film depends on the density of the volatileorganic solvent relative to water. A film with a three dimensionalnetwork of spherical pores as obtained by Yabu and Shimomura is formedif the solvent is less dense than water whereas a film containing onlysurface cavities is obtained if the solvent has a higher density thanwater. Srinivasarao mention polymers such as polystyrene containing anend-terminated carboxylic acid group, cellulose acetate andpolymethylmethacrylate as being suitable polymers for forming themicro-voided polymer films but do not mention any specific properties toguide the selection of polymers. Furthermore, Srinivasarao et al. do notteach how the micro-voided polymer films are to be used for preparingmicro-lens arrays for improved light extraction in OLEDs.

Copending U.S. Ser. No. 11/741,472, the disclosure of which isincorporated by reference herein, describes a simple method forpreparing microlens array films with hemispherical shaped microlensesand high fill factor. The process involves forming a solution of anorganic solvent polymer in a volatile water-immiscible organic solventhaving specific gravity greater than that of water, casting the solutionin a humid environment and condensing water droplets on the castsolution, evaporating off the solvent and condensed water droplets tocreate a first structured polymer film, coating a second fluid polymercomposition over the first structured polymer film, curing the secondpolymer fluid composition while it is in contact with the firststructured film to render it solid and create a second structured filmcomprising a first flat side and a second side with an array ofmicrolenses corresponding to the cavities in the first structured film,separating the second structured film from the first structured film andattaching the flat side of the second structured film to a transparentsubstrate of a light-emitting device. While the disclosed processproduces a microlens array with high fill factor and a randomdistribution of microlenses having nearly hemispherical shape, thematerial employed for formation of the second structured film in thedisclosed examples has a high hydrophobicity and low surface energy inorder to facilitate separation of the cured second structured film fromcontact with the first structured film. Elastomeric materials such assilicones or natural rubber that are hydrophobic and have low surfaceenergy, however, may not have desirable optical properties in terms ofrefractive index or light transmission or physical properties in termsof scratch resistance. While it is possible to chemically treat andmodify the surface of the first structured film (after it has beenprepared and before the second fluid composition is applied on it) toallow materials that are hydrophilic and high surface energy to bereleased from it, this involves several additional process steps thatare not desirable.

An object of this invention is to provide a light-emitting device, suchas an OLED device comprising a transparent substrate or cover throughwhich light is emitted, with a micro-lens array having refractive indexclosely matched to that of the transparent substrate or cover, whereinthe micro-lens array has a high fill factor of relatively smallmicrolenses that are randomly distributed, wherein the microlens arrayis obtainable by a simple low-cost method and the light emitting devicedemonstrates high light output.

SUMMARY OF THE INVENTION

In accordance with one embodiment, the invention is directed towards amethod of manufacturing a micro-lens array and light-emitting devicecomprising

-   -   a) forming a solution of an organic soluble copolymer or polymer        blend comprising a hydrophilic, high surface energy component in        combination with a hydrophobic, low surface energy component in        a volatile water-immiscible organic solvent having specific        gravity greater than that of water,    -   b) casting the solution in a humid environment, and condensing        water droplets on the cast solution,    -   c) evaporating off the solvent and condensed water droplets from        the cast composition to create a first structured polymer film        with close packed surface cavities having a mean diameter of        less than 20 micrometers,    -   d) annealing the first structured polymer film at elevated        temperature to achieve chemical re-construction to modify the        surface of the first structured polymer film and obtain a        relatively lower surface energy surface,    -   e) coating a second fluid polymer composition over the first        structured polymer film,    -   f) curing the second fluid polymer composition while it is still        in contact with the first structured polymer film to render it        solid and create a transparent second structured film comprising        a first flat side and a second side with an array of microlenses        formed thereon corresponding to the cavities of the first        structured film, wherein the second structured film comprises a        relatively high surface energy material and has a refractive        index greater than 1.45, and wherein the microlenses are        randomly distributed over 300 μm×300 μm areas of the second        structured film,    -   g) separating the second structured film with the micro-lens        array from the first structured polymer film, and    -   h) attaching the flat side of the second structured film to a        transparent substrate or cover of a light-emitting device        through which light is emitted.

In accordance with a further embodiment, the invention is directedtowards a light-emitting device, comprising: a light emitting element ona first side of a transparent substrate or cover through which light isemitted; and a microlens array on a second side, opposite to the firstside, of the transparent substrate or cover though which light isemitted; wherein the microlens array has a refractive index greater than1.45 and an elastic modulus of at least 1,000 kPa and comprisesindividual microlenses having a mean diameter of less than 20micrometers that are randomly distributed over 300 μm×300 μm areas ofthe microlens array.

The required first structured polymer film with cavities is formeddirectly upon evaporation of the organic solvent and condensed waterdroplets from the cast composition, and does not require additionalsteps as in prior art processes. Furthermore, chemical re-constructionof the surface of the first structured film is easily accomplished bysimply annealing the film at elevated temperature to create a relativelylower surface energy non-stick surface that allows materials withdesired optical and physical properties to be easily released from it. Amicro-lens array with the desired features in terms of high fill factor,high refractive index, high elastic modulus, and random distribution ofthe microlenses may be obtained from this micro-voided film in astraightforward manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a prior art organiclight-emitting diode (OLED) without a microlens array layer;

FIG. 2 shows a cross-sectional view of an OLED having a microlens arraylayer;

FIG. 3 shows a cross-sectional view of one embodiment of an OLEDprepared in accordance with the present invention;

FIG. 4 shows the EL spectra of Devices 1, 2, and 3 of Examples 3, 4, and5 tested at 1.0 mA/cm²;

FIG. 5 shows the angular dependent luminescence of Devices 1, 2, and 3Examples 3, 4, and 5 tested at 1.0 mA/cm²;

FIG. 6 shows an image of the cross-section of the first structuredpolymer film prior to annealing in an embodiment of the invention;

FIG. 7 shows an image of the cross-section of the first structuredpolymer film after annealing in an embodiment of the invention; and

FIG. 8 shows an image of the cross-section of the second structuredpolymer film or microlens array in an embodiment of the invention.

It will be understood that FIGS. 1-3 are not to scale since theindividual layers are too thin and the thickness differences of variouslayers are too great to permit depiction to scale.

DETAILED DESCRIPTION

Microlens arrays as employed in combination with light emitting devicesin the present invention may be formed by forming a solution of anorganic soluble polymer comprising a copolymer or polymer blend withdefined surface energy and wetting properties, preferably along with afluorocarbon surfactant, in a volatile water-immiscible organic solventhaving density greater than that of water, casting this solution in ahumid environment, evaporating off the solvent and condensed waterdroplets from the cast composition to create a first structured polymerfilm with surface cavities, annealing the first structured polymer filmat elevated temperature to achieve chemical re-construction of thesurface of the film, coating a second fluid polymer composition over thefirst structured polymer film, curing the second fluid polymercomposition while it is still in contact with the first structuredpolymer film to render it solid and create a transparent secondstructured film comprising a first flat side and a second side with anarray of microlenses formed thereon corresponding to the cavities of thefirst structured film, and separating the second structured film withthe micro-lens array from the first structured polymer film. Theresulting microlens array may then be attached to a transparentsubstrate or cover of a light emitting device through which light isemitted, such as an OLED device, to form a device in accordance with theinvention. As the first structured polymer film is directly formed froma copolymer or polymer blend that forms a relatively low surface energysurface upon annealing, the microlens array may be formed with arelatively higher surface energy material, thereby also enabling use ofa material having a desirably high refractive index such that it may beclosely matched to that of the substrate or cover of the light-emittingdevice. Such relatively higher surface energy materials may also have adesirably high elastic modulus, providing improved scratch resistance.

Suitable materials for the organic soluble polymer used for preparingthe first structured polymer film are copolymers or polymer blendscomprising a mixture of a hydrophilic (and high surface energy)component and a hydrophobic (and low surface energy) component. Thesurface energy of the hydrophilic (and high surface energy component) issuitably greater than 30 dynes/cm and more suitably greater than 35dynes/cm. The contact angle that a drop of water makes with the surfaceof the hydrophilic component is suitably between 50 and 90 degrees andmore suitably between 60 and 80 degrees. The surface energy of thehydrophobic (and low surface energy) component is suitably less than 30dynes/cm and more suitably less than 25 dynes/cm to allow a secondstructured polymer film to be easily released from it by peeling off andenable the creation of a microlens array with individual lenses closelyconforming to the tightly packed arrangement of the cavities of thefirst structured film employed as a mold. It is also desired that thesurface properties of the hydrophobic component is such that a drop ofwater placed on it will have a contact angle in excess of 90 degrees.

Suitable materials for the organic soluble polymer are copolymerswherein one of the components of the copolymer is a hydrophilic segmentsuch as polystyrene, polyester, polyethyleneterephthalate (PET),polyvinylchloride (PVC), polycarbonate, polyimide, polysulfone,polyethersulfone, bisphenol A polycarbonate (BPAC) or nylon and thesecond component is a hydrophobic segment such as polydimethylsiloxane(PDMS) or a fluorinated material such as fluorinated ethylene propyleneor polyvinylfluoride. The copolymers may be either random or blockcopolymers such as di-block or tri-block copolymers. Particularlysuitable are copolymers containing polystyrene and PDMS or copolymerscontaining bisphenol A polycarbonate (BPAC) and PDMS. Each PDMS block inthe copolymer suitably contains between 2 and 40 repeat units ofdimethylsiloxane and more suitably between 15 and 25 units ofdimethylsiloxane. The proportion of PDMS in the copolymer is suitablybetween 0.5 mol % and 20 mol % and more suitably between 1 mol % and 10mol %. The average molecular weight of the copolymer is suitably in therange of 20,000 to 500,000.

Also suitable as materials for the organic soluble polymer are blends ofa hydrophilic polymer such as polystyrene, polyester,polyethyleneterephthalate (PET), polyvinylchloride (PVC), polycarbonate,polyimide, polysulfone, polyethersulfone, bisphenol A polycarbonate(BPAC) or nylon and a hydrophobic polymer such as polydimethylsiloxane(PDMS) or a fluorinated material such as fluorinated ethylene propyleneor polyvinylfluoride. Most suitable are blends of polystyrene and PDMSor polycarbonate and PDMS. The proportion of PDMS in the blend issuitably between 0.5 mol % and 20 mol % and more suitably between 1 mol% and 10 mol %.

To facilitate formation of closely packed hemispherical structures inthe surface of the first structured polymeric film, a fluorocarbonsurfactant is preferably used in conjunction with the organic solublepolymer in the method of the invention. Fluorocarbon surfactants are aclass of surfactants wherein the hydrophobic part of the amphiphilecomprises at least in part some portion of a carbon-based linear orcyclic moiety having fluorines attached to the carbon where typicallyhydrogens would be attached to the carbons together with a hydrophilichead group. Some typical non-limiting fluorocarbon surfactants includefluorinated alkyl polyoxyalkylene and fluorinated alkyl esters as wellas ionic surfactants. Representative structures for these compounds aregiven below:

-   -   (I) R_(f)R(R₁O)_(x)R₂    -   (II) R_(f)R—OC(O)R₃    -   (III) R_(f)R—Y—Z    -   (IV) R_(f)RZ        wherein R_(f) contains from 6 to about 18 carbons each having        from about 0 to about 3 fluorines attached. R is either an alkyl        or alkylene oxide group which when present, has from about 1 to        about 10 carbons and R₁ represents an alkylene radical having        from about 1 to 4 carbons. R₂ is either a hydrogen or a small        alkyl capping group having from 1 to about 3 carbons. R₃        represents a hydrocarbon moiety comprising from about 2 to 22        carbons including the carbon on the ester group. This        hydrocarbon can be linear, branched or cyclic saturated or        unsaturated and may contain moieties based on oxygen, nitrogen        and sulfur including, but not limited to ethers, alcohols,        esters, carboxylates, amides, amines, thio-esters, and thiols;        these oxygen, nitrogen and sulfur moieties can either interrupt        the hydrocarbon chain or be pendant on the hydrocarbon chain. In        the third structure above, Y represents a hydrocarbon group that        can be an alkyl, pyridine group, amidopropyl, etc that acts as a        linking group between the fluorinated chain and the hydrophilic        head group. In the third and fourth structures, Z represents a        cationic, anionic and amphoteric hydrophilic head group        including but not limited to carboxylates, sulfates, sulfonates,        quaternary ammonium groups and betaines. Non-limiting        commercially available examples of these structures include        Zonyl 9075, FSO, FSN, FS-300, FS-310, FSN-100, FSO-100, FTS and        TBC from DuPont and Fluorad surfactants FC430, FC431, FC-740,        FC-99, FC-120, FC-754, FC-170C and FC-171 from 3M in St. Paul,        Minn.

Suitable solvents for the organic soluble polymer and the fluorocarbonsurfactant are halogen based organic solvents such as chloroform,dichloromethane and dichloroethane; aromatic hydrocarbons such asbenzene, toluene and xylene; esters such as ethyl acetate and butylacetate; water insoluble ketones such as methyl isobutyl ketone; andcarbon disulfide. The organic solvents may be used alone or in the formof a mixed solvent comprising a combination of two or more. In order toform hemispherical cavities only on the surface of the first structuredfilm, the organic solvent should have a specific gravity greater than1.0 and more suitably greater than 1.2. Also, it is preferred that theboiling point of the organic solvent is less than 120 C at normalatmospheric pressure and more suitably less than 100 C. Furthermore, thelatent heat of evaporation of the organic solvent is desirably greaterthan 200 kJ/kg and more desirably greater than 300 kJ/kg. It is alsodesired that the solubility of the organic solvent in water at roomtemperature is less than 5 g/100 mL and more suitably less than 2 g/100mL.

The concentration of the organic soluble polymer in the organic solventis suitably between 5 wt % and 40 wt % and the concentration of thefluorocarbon surfactant is most suitably less than 1.0 wt % based on theweight of the organic soluble polymer, preferably from 0.05 to 0.5% andmore preferably 0.05 to 0.2% based on the concentration of polymer.

The cast solution of the organic soluble polymer and the fluorocarbonsurfactant in the organic solvent is most suitably exposed to humid airin a humidity chamber where the relative humidity is controlled between40 and 95%. After evaporation of the organic solvent and the condensedwater droplets, the resulting hemispherical surface cavity structuredpolymer film preferably has a surface fluorine content between 2 and 20atom % and more suitably between 3 and 10 atom %.

The use of a fluorocarbon surfactant, along with other materialselections and process conditions in accordance with the abovedescriptions, has been found to be particularly effective for enablingformation of a first structured film having a high fill factor (greaterthan 80%, preferably greater than 85%) of relatively small (less than 20micrometer, preferably 1-10 micrometer diameter) cavities for use as amold in forming a microlens array for use in the invention.

The first structured polymer film may be annealed in air by heating to atemperature close to or above the Tg of the major component of thecopolymer or polymer blend. Upon annealing in dry air, the lower surfaceenergy hydrophobic component preferentially migrates to the surface tomodify the top surface of the first structured film to a hydrophobicsurface. A film with surface cavities and hydrophobic surface isobtained in this manner. The hydrophobic top surface exhibits improvednonstick properties and can be used as a mold having low surface energy.While cavities are preferably initially formed in the first structuredfilm with hemispherical shapes (having a mean cavity depth to diameterratio of greater than 0.30, preferably greater than 0.35 and mostpreferably greater than 0.40), such cavities may become slightly lesshemispherical upon annealing.

Where PDMS and BPAC are employed as hydropobic and hydrophiliccomponents, respectively, the degree of hydrophobicity may be expressedas an atomic ratio of Si/C at the surface where the ratio is 0.5 maximumfor all PDMS at the surface (Si(CH₃)₂) and maximum hydrophobicity andthe ratio is 0.0 for no silicon at the surface. Si/C ratios of less than0.3 may be regarded as relatively hydrophilic while ratios of 0.3 orgreater may be regarded as relatively hydrophobic.

A second fluid polymer composition may be coated over the mold, curedand released to form the microlens array. The hydrophobic top surface ofthe mold allows a variety of hydrophilic (and high surface energy)materials to be easily released from it. Suitable materials for thesecond fluid polymer composition are ultraviolet (UV) curable materialssuch as UV curable acrylates, urethanes and urethane acrylate oligomers.Examples of such materials are NOA 68 and NOA 72 from Norland ProductsInc., AC PR-153, AC PR-155, AC PR-157 and AC PR-157-S5 from AddisonClear Wave and CN 968 from the Sartomer Company Inc. Particularlysuitable are fluid polymer compositions that after curing and releaseresult in a second structured film having a refractive index greaterthan 1.45, an elastic modulus greater than 1000 kPa and opticaltransmission greater than 80%. More desirable are fluid polymercompositions that after curing and release result in a second structuredfilm having a refractive index greater than 1.5, an elastic modulusgreater than 10,000 kPa and optical transmission greater than 85%. Mostdesirable are fluid polymer compositions that after curing and releaseresult in a second structured film having refractive index greater than1.5, an elastic modulus greater than 100,000 kPa and opticaltransmission greater than 90%. It is desired that the refractive indexof the second structured film is closely matched to that of atransparent glass substrate or cover through which light is emitted froma light emitting device (e.g., where the refractive index of the secondstructured film is within 0.05, preferably within 0.03, and mostpreferably within 0.01 of the refractive index of the cover or substratethrough which light is emitted). Preferably, the microlens array isadhered to the transparent substrate or cover through which light isemitted with an optical adhesive having a relatively high refractiveindex, which is also closely matched to that of the transparentsubstrate or cover.

The described method enables formation of a second structured filmhaving a first flat side and a second side having a high fill factor(greater than 80%, preferably greater than 85%) of relatively small(less than 20 micrometer, preferably 1-10 micrometer diameter)microlenses corresponding to the cavities formed in the first structuredfilm mold. The microlenses formed in the second structured film inaccordance with the process of the invention are close-packed to providethe high fill factor, but disordered or randomly distributed. The degreeof disorder in the film is evident in the Fraunhofer diffraction patternthat is exhibited by the film when the film is placed before a source oflaser light. The diffraction patterns exhibited by the random micro-lensarray films employed in the invention when illuminating the micro-lensarray with a beam of light typically comprise one or more distinctconcentric rings, whereas the diffraction patterns of films comprisingan ordered array of lenses will be in the form of individual brightpoints. The phenomenon of Fraunhofer diffraction is described more fullyby Lisensky et al. Journal of Chemical Education, vol. 68, February1991. When used in a pixellated flat-panel display device, it isimportant that the distribution of microlenses is random at the scale ofa pixel in the display for the desirable effects of the invention to beapparent to the viewer. Pixel sizes in typical displays range from 50μm×50 μm to 300 μm×300 μm. The process of the present inventionadvantageously enables close packed microlenses providing a high fillfactor wherein the microlenses are randomly distributed over such acorresponding 300 μm×300 μm area of the second structured film.

The microlens array of the second structured film formed in the methodof the present invention can be integrated into a light-emitting devicesuch as a flat-panel pixellated display by adhering the flat side of thesecond structured film to either a transparent cover or substrate of thelight emitting device through which light is emitted. In a preferredembodiment, the light-emitting element is an OLED comprising first andsecond electrodes and one or more layers of organic light-emittingmaterial formed between the electrodes, wherein at least one electrodecomprises a transparent electrode and is positioned between the organiclayers and the transparent substrate or cover through which light isemitted.

The present invention may also be practiced with either active- orpassive-matrix light emitting devices. It may also be employed indisplay devices or in area illumination devices. In a preferredembodiment, the present invention is employed in a flat-panel OLEDdevice composed of small molecule or polymeric OLEDs as disclosed in butnot limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang etal., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke etal. Many combinations and variations of organic light-emitting displayscan be used to fabricate such a device, including both active- andpassive-matrix OLED displays having either a top- or bottom-emitterarchitecture. While the invention is described primarily with respect toorganic light emitting diode devices, it is not limited to such OLEDdevices, and may also be employed with other self-emissivelight-emitting devices such as inorganic LEDs.

A typical structure according to a prior art device is shown in FIG. 1.OLED 100 in FIG. 1 includes transparent glass substrate 110.2, anode110.3, hole-injecting layer (HIL) 120, hole-transporting layer (HTL)130, exciton-blocking layer (XBL) 140, light-emitting layer (LEL) 150,electron-transporting layer (ETL) 170, electron-injecting layer (EIL)180 and cathode 190. OLED 200 shown in FIG. 2 is the same as OLED 100,except that there is a microlens array layer 210.1 (microlens arraylayer 1) disposed on the side of the transparent glass substrate 110.2opposite to the device side. A structure according to one embodiment ofthe present invention is shown in FIG. 3. OLED 300 is the same as OLED100, except that there is a microlens array layer 310.1 (microlens arraylayer 2) disposed on the side of the transparent glass substrate 110.2opposite to the device side. The microlens array layer 310.1 isfabricated in accordance with the present invention. OLEDS 100, 200, and300 can be operated by applying an electric potential produced by avoltage/current source between the pair of the electrodes, anode 110.2and cathode 190. Light generated inside the devices emits through thetransparent glass side. The composition of each organic layer in theOLEDs can be as taught in the prior art.

The micro-lens array of the present invention provides improveddiffusion of ambient light. This is advantageous when concentratedintense light sources such as the sun or incandescent lamps are incidenton the display. The specular reflections of such sources detract fromthe quality of the viewed image. Furthermore, the disordered nature ofthe micro-lenses in the present invention serves to minimize undesirablediffractive effects from such concentrated intense light sources, whichcreate bright reflected spots or rings that are distracting to theviewer.

Most OLED devices are sensitive to moisture or oxygen, or both, so theyare commonly sealed in an inert atmosphere such as nitrogen or argon,along with a desiccant such as alumina, bauxite, calcium sulfate, clays,silica gel, zeolites, alkaline metal oxides, alkaline earth metaloxides, sulfates, or metal halides and perchlorates. Methods forencapsulation and desiccation include, but are not limited to thosedescribed in U.S. Pat. No. 6,226,890 issued May 8, 2001 to Boroson etal. In addition, barrier layers such as SiO_(x)(x>1), Teflon, andalternating inorganic/polymeric layers are known in the art forencapsulation.

OLED devices can employ various well-known optical effects in order toenhance their properties if desired. This includes optimizing layerthicknesses to yield maximum light transmission, providing dielectricmirror structures, replacing reflective electrodes with light-absorbingelectrodes, providing anti-glare or anti-reflection coatings over thedisplay, providing a polarizing medium over the display, or providingcolored, neutral density, or color conversion filters over the display.Filters, polarizers, and anti-glare or anti-reflection coatings may bespecifically provided over the cover or as part of the cover.

EXAMPLES

The following examples are presented for a further understanding of thepresent invention. During the fabrication of OLEDs, the thickness of theorganic layers and the doping concentrations were controlled andmeasured in situ using calibrated thickness monitors built in anevaporation system (Made by Trovato Mfg., Inc., Fairport, N.Y.). The ELcharacteristics of all the fabricated devices were evaluated using aconstant current source (KEITHLEY 2400 SourceMeter, made by KeithleyInstruments, Inc., Cleveland, Ohio) and a photometer (PHOTO RESEARCHSpectraScan PR 650, made by Photo Research, Inc., Chatsworth, Calif.) atroom temperature and 1.0 mA/cm². The color was reported using CommissionInternationale de l'Eclairage (CIE) coordinates.

EXAMPLE 1 Microlens Array

This example illustrates a method of preparation of a microlens arrayemployed in accordance with an embodiment of the invention.

A solution containing 20 wt % of a copolymer of bisphenol Apolycarbonate (BPAC) and polydimethylsiloxane (PDMS) (49.5 mol %bisphenol A, 50 mol % polycarbonate and 1 mol % PDMS; average molecularweight of 92,000 and 31 repeat units of dimethylsiloxane in each PDMSblock) and 0.1% FC431 fluorocarbon surfactant (based on the weight ofpolymer) in methylene chloride was applied on the vinylidene chloridesubbed surface of a sheet of polyethyleneterephthalate (PET) at a wetthickness of 0.25 mm. The wet coating was then immediately inserted intoa closed chamber where the temperature and humidity were controlled at22 C and 65% RH and kept there for 5 minutes. After evaporation of theorganic solvent, the sheet was kept in the chamber for an additional 5minutes and the chamber was purged with nitrogen gas to remove residualwater. The dried polymer film was peeled off the PET substrate. The filmwas structured with closely packed hemispherical cavities. The film wasannealed for 30 minutes in a heated environment maintained at 120 C.Analysis of the first 34 angstroms of the surface of the film before andafter heating by X-ray photoelectron spectroscopy (XPS) showed that theratio of the atom % silicon to the atom % carbon had increased from 0.26to 0.32 indicating a higher fraction of the more hydrophobic PDMS at thesurface of the film after annealing. Examination of the cavities in thefilm before and after heating by scanning electron microscopy (SEM)showed that the original hemispherical shaped cavities were shallowerafter heating (FIG. 6 and FIG. 7).

A UV curable polyurethane material NOA 68 was then applied to thesurface of the micro-voided film at a wet thickness of 0.2 mm andsubsequently cured using UV radiation of 1.0 joule/cm². The hydrophiliccured material (water contact angle of 70 degrees) was easily peeled offthe mold to provide a random array of microlenses of refractive index1.54 that is close to that of the glass substrate of an OLED device(1.52) and relatively high elastic modulus (138,000 kPa). FIG. 8 showsan SEM of the microlens array film. Examination of the film using laserlight showed a ring diffraction pattern characteristic of a randompattern of microlenses.

A control experiment was done with a first structured film that had notbeen annealed. It was found that in this case the hydrophilic secondpolymer composition could not be released from the first structured filmafter it had been cured.

EXAMPLE 2 Microlens Array

This example illustrates preparation of a microlens array employed in acomparative example.

A solution containing 23.8 wt % polycarbonate (Bayer DPI 1265) and 0.1%FC431 fluorocarbon surfactant (based on the weight of polycarbonate) indichloromethane was applied on the surface of a sheet ofpolyethyleneterephthalate (PET) at a coverage of 452 cm³/m². The wetcoating was then immediately inserted into a closed chamber where thetemperature and humidity were controlled at 22 C and 85% RH and keptthere for 5 minutes. After evaporation of the organic solvent, the sheetwas kept in the chamber for an additional 5 minutes and the chamber waspurged with nitrogen gas to remove residual water. The resulting driedpolymer film was then peeled off the PET substrate. The film wasstructured with closely packed hemispherical cavities. Sylgard 184silicone elastomer base (from Dow Coming Corporation) was combined withSylgard curing agent (also from Dow Coming) in a 10:1 weight ratio. Themixture was applied at a coverage of 151 cm³/m² to the surface of thehemispherical cavities structured polymer film at 22 C. The elastomerwas then cured in contact with the mold by heating to 100 C for onehour. The cured elastomer film (with surface energy of 23 dynes/cm,aqueous contact angle of 105° and refractive index of 1.41) with theimprint of the mold was released from the mold by peeling off to createthe micro-lens array.

EXAMPLE 3 Comparative Device

The preparation of a conventional OLED (Device 1) is as follows: A ˜1.1mm thick glass substrate (refractive index 1.52) coated with atransparent indium-tin-oxide (ITO) conductive layer was cleaned anddried using a commercial glass scrubber tool. The thickness of ITO isabout 100 nm and the sheet resistance of the ITO is about 30 Ω/square.The ITO surface was subsequently treated with oxidative plasma tocondition the surface as an anode. A layer of CF_(x), 1 nm thick, wasdeposited on the clean ITO surface as the anode buffer layer bydecomposing CHF₃ gas in an RF plasma treatment chamber. The substratewas then transferred into a vacuum deposition chamber for deposition ofall other layers on top of the substrate. The following layers weredeposited in the following sequence by evaporation from a heated boatunder a vacuum of approximately 10⁻⁶ Torr:

-   -   a) a hole-injecting layer (HIL), 10 nm thick, including        hexaazatriphenylene hexacarbonitrile (HAT-CN);    -   b) a hole-transporting layer (HTL), 75 nm thick, including        N,N′-di-1-naphthyl-N,N′-diphenyl-4,4′-diaminobiphenyl (NPB);    -   c) an excition-blocking layer (XBL), 10 nm thick, including        4-(9H-carbazol-9-yl)-N,N-bis[4-(9H-carbazol-9-yl)phenyl]-benzenamine        (TCTA);    -   d) a light-emitting layer (LEL), 20 nm thick, including        2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole] (TPBI)        as a host material and including tris(1-phenylisoquinoline)        iridium (III) (Ir(piq)₃) as a dopant material, wherein the        dopant concentration in the LEL is about 4% by volume;    -   e) an electron-transporting layer (ETL), 15 nm thick, including        4,7-diphenyl-1,10-phenanthroline(Bphen);    -   f) an electron-injecting layer (EIL), 40 nm thick, including        Bphen doped with 0.8% lithium by volume; and    -   g) a cathode: approximately 100 nm thick, including Al.

After the deposition of these layers, the device was transferred fromthe deposition chamber into a dry box (made by VAC Vacuum AtmosphereCompany, Hawthorne, Calif.) for encapsulation. The OLED has anrectangular emission area of 2.66 cm².

Device 1 is denoted as: Glass/ITO/10 nm HAT-CN/75 nm NPB/10 nm TCTA/20nm TPBI:4% Ir(piq)₃/15 nm Bphen/40 nm Bphen:0.8% Li/100 nm Al. The ELperformance of the device is summarized in Table 1 and its EL spectrumis shown in FIG. 4. The angular dependent luminance of the device isshown in FIG. 5.

EXAMPLE 4 Comparative Device

Another OLED (Device 2) is the same as Device 1 of Example 3, exceptthat a microlens array layer prepared as described in Example 2 (withrefractive index 1.41) is attached to the glass surface as shown in FIG.2.

Device 2 is denoted as: Microlens array layer 1/Glass/ITO/10 nmHAT-CN/75 nm NPB/10 nm TCTA/20 nm TPBI:4% Ir(piq)₃/15 nm Bphen/40 nmBphen:0.8% Li/100 nm Al. The EL performance of the device is summarizedin Table 1 and its EL spectrum is shown in FIG. 4. The angular dependentluminance of the device is shown in FIG. 5.

EXAMPLE 5 Invention Device

Another OLED (Device 3) is the same as Device 1 of Example 3, exceptthat a microlens array layer formed in accordance with the presentinvention as described in Example 1 (with refractive index 1.54, closelymatched to that of the device glass substrate) is attached to the glasssurface as shown in FIG. 3.

Device 3 is denoted as: Microlens array layer 2/Glass/ITO/10 nmHAT-CN/75 nm NPB/10 nm TCTA/20 nm TPBI:4% Ir(piq)₃/15 nm Bphen/40 nmBphen:0.8% Li/100 nm Al. The EL performance of the device is summarizedin Table 1 and its EL spectrum is shown in FIG. 4. The angular dependentluminance of the device is shown in FIG. 5.

Comparing to Device 2, the light output of Device 3 having a newmicrolens array layer is improved, even though the microlenses of thecomparison film formed in Example 2 where hemispherical while thecavities formed in the first structured film after heating wereshallower than the original hemispherical shaped cavities formed in thefilm before heating (as noted in Example 1).

TABLE 1 Luminous CIE x Power ΔEQE** Example Voltage Luminance EfficiencyCIE y Efficiency EQE* (On- ΔEQE*** (Type) (V) (cd/m²) (cd/A) (1931)(lm/W) (%) Axis) (Integrated) 3 (Comp) 3.7 125 12.5 0.673 10.7 15.30.326 4 (Comp) 3.7 156 15.6 0.671 13.3 18.3 19.6%   28% 0.328 5 (Inv)3.7 163 16.3 0.671 14.4 19.0 24.2% 29.3% 0.328 *On-axis external quantumefficiency (EQE) **ΔEQE (On-Axis) = (EQE of device 2 or 3 − EQE ofdevice 1)/EQE of device 1 (on-axis) ***ΔEQE (Integrated) is a integratedvalue of ΔEQE from 0 to 90 degree related to on-axis.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A light-emitting device, comprising: a light emitting element on afirst side of a transparent substrate or cover through which light isemitted; and a microlens array on a second side, opposite to the firstside, of the transparent substrate or cover though which light isemitted; wherein the microlens array has a refractive index greater than1.45 and an elastic modulus of at least 1,000 kPa and comprisesindividual microlenses having a mean diameter of less than 20micrometers that are randomly distributed over 300 μm×300 μm areas ofthe microlens array.
 2. A light emitting device according to claim 1,wherein the device comprises a pixellated flat-panel display device, andwhere the individual micro-lenses of the array are randomly distributedat the scale of a pixel in the display device.
 3. The light emittingdevice of claim 1, wherein the microlens array has a microlens fillfactor greater than 0.80.
 4. The light emitting device of claim 1,wherein the device comprise, in order, a transparent glass substratethrough which light is emitted, a first transparent electrode on a firstside of the substrate, one or more layers of organic light-emittingmaterial, and a second electrode, and wherein the microlens array is ona second side, opposite to the first side, of the transparent substrate,and has an refractive index within 0.05 of the refractive index of theglass substrate.
 5. The light emitting device of claim 4, wherein themicrolens array has an refractive index within 0.03 of the refractiveindex of the glass substrate.
 6. The light emitting device according toclaim 5, wherein the elastic modulus of the second structured film is atleast 10,000 kPa.